1. Field of the Invention
The present invention relates to a bearing unit for use in a table saddle of a machine tool or a sliding part of a machining center or a transfer system.
2. Description of the Prior Art
A typical conventional bearing unit has heretofore been arranged as shown in FIG. 26. More specifically, a track rail 101 of continuous length is provided on one of its sides with a ridge 101a which extends in the longitudinal direction thereof. On both sides of the ridge 101a are formed two ball-rolling surfaces 102 which extend in the longitudinal direction of the track rail 101. On the other hand, a bearing body 103, which slides along the track rail 101 in the longitudinal direction thereof, is provided on one of its side surfaces with a recess 103a extending in the longitudinal direction of the bearing body 103 while facing the ridge 101a on the track rail 101. On both sides of the recess 103a are formed two ball-rolling surfaces 104, 104 which respectively face the ball-rolling surfaces 102, 102 formed on both sides of the ridge 101a on the track rail 101. Further, a multiplicity of balls 105, 105 . . . are interposed between the opposing ball-rolling surfaces 102, 102 and 104, 104, whereby the bearing body 103 is lightly slid along the track rail 101 by virtue of the rolling of these balls 105, 105 . . . . In addition, in order to recirculate the balls 105, 105 . . . , the bearing body 103, which is solid, has two non-loaded ball rolling bores 107, 107 extending therethrough, the non-loaded ball rolling bores 107, 107 being continuous with two loaded ball rolling passages 106, 106 which are respectively constituted by the opposing ball-rolling surfaces 102, 102 and 104, 104, whereby two endless ball recirculating passages are respectively constituted by the loaded ball rolling passages 106, 106 and the non-loaded ball rolling bores 107, 107 which correspond to each other. Moreover, the balls 105, 105 . . . within the loaded ball rolling passages 106, 106 are guided by a retainer 109 which is formed with slits 108, 108 extending in the longitudinal direction thereof in such a manner that the balls 105, 105 . . . properly roll within the loaded ball rolling passages 106, 106, and furthermore are prevented from coming off.
In this type of conventional bearing unit, the non-loaded ball rolling bores 107, 107 are longitudinally formed in the solid bearing body 103 by means of, for example, a drill. For this reason, as the length of the bearing body 103 increases, it becomes difficult to effect accurate boring due to, for example, the deflection of the drill. Further, when boring is effected from both end surfaces of the bearing body 103 in such a manner that the bores being cut from both end surfaces join together at an intermediate portion inside the bearing body 103 to thereby form one through-bore, an undesirable step is easily formed at the intermediate portion where they join. In order to eliminate such a step, it is inconveniently necessary to conduct such a correcting operation as forming a larger bore again or reaming. Accordingly, much time and labor are required to carry out the boring operation, which fact constitutes a factor in the increase of costs.
Further, since the length of the bearing body 103 is thus limited, the length of each of the loaded ball rolling passages 106, 106 is inevitably limited also. It is, therefore, extremely difficult to increase the length of each of the loaded ball rolling passages 106, 106 for the purpose of increasing the rated load-bearing capacity of the bearing beyond a predetermined limit.
Moreover, if the length of each of the ball-rolling surfaces 104, 104 on the bearing body 103 is changed for the purpose of changing the rated load-bearing capacity of the bearing, it is necessary to change the length of the retainer 109 correspondingly. In such a case, it is further necessary to change the length of each of the slits 108, 108 of the retainer 109 and the position of a ball-guiding tongue (not shown) which projects from each of the ends of each slit 108. For this reason, it is necessary to change the arrangement of the retainer 109 in accordance with a change in the rated load-bearing capacity of the bearing so that the retainer 109 possesses slits of proper length and tongues located at proper positions, which fact also leads to an increase in cost.
In the conventional bearing unit, further, the ball-rolling surfaces 104, 104 are formed on both sides of the recess 103a of the bearing body 103, which involves a narrow distance d between the ball-rolling surfaces 104, 104 and bolt receiving bores 110 formed in the bearing body 103. As a result, when the ball-rolling surfaces 104, 104 are hardened by, for example, high-frequency (induction) hardening for the purpose of increasing wear resistance, the thin-walled portion between the ball-rolling surfaces 104, 104 and the bolt receiving bores 110 may disadvantageously melt on heating at high temperature. For this reason, it is not possible to reduce the distance d between the bolt receiving bores 110 and the ball-rolling surfaces 104, 104 by a large margin. Accordingly, the reduction in the size and weight of the bearing body 103 and, hence, of the bearing unit as a whole is greatly limited.
In addition, the ball-rolling surfaces 102, 102 of the track rail 101 are formed on both sides of the ridge 101a in such a manner that the ball-rolling surfaces 102, 102 extend horizontally. For this reason, dust or other foreign matter in the air may adhere to the ball-rolling surfaces 102, 102, resulting disadvantageously in a possible interference with the smooth sliding of the bearing body 103 in relation to the track rail 101.